1. Field of the Invention
The present invention relates to the technology of locating the position of an operation performed in a manufacturing process of a product.
2. Description of the Related Art
In a manufacturing process of a product requiring a number of operations to be performed by workers, there often occur erroneous operations by a worker forgetting an operation or failing to complete an operation. For example, the manufacturing process of a large electronic equipment unit, a printed circuit board unit, a communication equipment unit, etc. includes an assembly process of parts, and the assembly process includes a number of screwing operations for attaching a part to another part. In the assembly process, erroneous operations frequently occur from screwing operations.
Examples of erroneous operations are stopping a screwing operation before reaching a specified torque of a screw, forgetting to complete a screwing operation after a temporary screwing operation, forgetting to place a screw at a position where the screwing operation is to be performed, etc. Since there are various types of erroneous operations, there are countermeasures to be taken from various viewpoints to reduce the erroneous operations
For example, to tighten a screw with a specified torque, it is effective to use a device called a torque driver designed for starting idle running when an applied torque is detected to reach a specified torque. The device is used in an assembly process by a worker or in an assembly process by a robot. The patent document 1 describes a thread fastening device (namely, screwing device) for an assembly process by a robot. The thread fastening device is fixed to the hand of the robot, and detects the coordinates of a screw hole by recognizing an image. When a robot controller corrects the position of the robot based on the coordinates, the thread fastening device drives a nut runner, and tightens the screw while monitoring the applied torque and the driving time.
To reduce erroneous operations of forgetting an operation to be performed, there have conventionally been operation tools such as a screwing counter, an electronic shelf, etc. By using these operation tools, it is possible to compare the number of operations to be performed with the number of operations practically performed and detect forgetting an operation. However, it is not possible only by comparing the numbers to concretely determine which operation has been forgotten. Therefore, it is necessary to visually confirm the point where the screwing operation has been forgotten when inconsistency is detected as a result of the comparing the numbers. At present, there are still a number of erroneous operations even using these operation tools. As a result, there frequently occurs defective manufacture, which requires inspection and repair within a manufacturing process.
Thus, in a manufacturing process to be performed by a number of operations by workers, forgetting operations is a serious problem. The patent document 2 describes a work quality control system for solving the above-mentioned problem. In this work quality control system, a plurality of receivers receive a signal issued by an ultrasonic transmitter attached to the tip of a nut runner. Based on the difference in propagating time of a signal and the positional relation between receivers, the position of the transmitter is located. Since the plurality of receivers are set such that they can keep a predetermined relative positions from a workpiece to be assembled, the position of a tightened screw is detected from the position of the transmitter based on the relationship between the static relative positions. The location information indicating the thus detected position of the tightened screw and the fastening information indicating a result of the comparison between the actually applied torque and the specified torque are managed in combination.    [Patent Document 1] Japanese Published Patent Application No. H6-23631    [Patent Document 2] Japanese Published Patent Application No. 2004-258855
However, there are the following two problems with the system described in the patent document 2.
The first problem is that it is necessary to attach a transmitter of ultrasonic signal or radio signal to a tool. Such necessity cause a problem because the transmitter requires more power, the wiring for supply of power to the transmitter can be the obstacle to an operation, and/or the transmitter can not be attached to some types of existing tools. It is important for an invention for reducing forgetting an operation in a manufacturing process to be easily implemented in the existing manufacturing process.
The second problem is an assumption that it is predetermined that the positional relation between a workpiece and receivers is statically determined. In the product manufacturing process, there are process steps in which the position and the direction of a product in manufacturing is not predetermined. In some process steps, the position and the direction of a product are uncertain until a practical operation is performed due to various reasons including the convenience of a worker, etc. In the case in which one process includes a plurality of operations, the process can be performed with a different position or direction depending on each operation.